The present invention relates to a helically-shaped intake port of an internal-combustion engine.
A helically-shaped intake port normally comprises a helical portion formed around the intake valve of an engine and a substantially straight inlet passage portion tangentially connected to the helical portion. However, if such a helically-shaped intake port is so formed that a strong swirling motion is created in the combustion chamber of an engine when the engine is operating at a low speed under a light load, that is, when the amount of air fed into the cylinder of the engine is small, since air flowing within the helically-shaped intake port is subjected to a great flow resistance, a problem occurs in that the volumetric efficiency is reduced when the engine is operating at a high speed under a heavy load, that is, when the amount of air fed into the cylinder of the engine is large.
In order to eliminate such a problem, the present inventors previously proposed a helically-shaped intake port in which a bypass passage, branched off from the inlet passage portion and connected to the helix-terminating portion of the helical portion, is formed in the cylinder head of an engine. A flow control valve is arranged in the bypass passage and is open when the engine is operating under a heavy load at a high speed. In this helically-shaped intake port, when the engine is operating under a heavy load at a high speed, a part of the air introduced into the inlet passage portion is fed into the helical portion of the helically-shaped intake port via the bypass passage. Consequently, since the flow area of the intake port is increased when the engine is operating under a heavy load at a high speed, it is possible to increase the volumetric efficiency. However, in this helically-shaped intake port, since the bypass passage is formed by a tubular passage which is completely separated from the inlet passage portion, the bypass passage has a relatively great flow resistance. In addition, since it is necessary to form the bypass passage adjacent to the inlet passage portion, the cross-sectional area of the bypass passage is restricted by the presence of the inlet passage portion. Consequently, it is difficult to obtain a satisfactory high volumetric efficiency. In addition, the helically-shaped intake port has a complicated construction itself, and, thus, if a bypass passage completely separated from the inlet passage portion is additionally provided, the entire construction of the intake port becomes extremely complicated. Therefore, it is considerably difficult to form a helically-shaped intake port equipped with such a bypass passage in the cylinder head.